Add support for bash commands

[cawoodm]

With the Python Core/Kernel it's possible to run bash commands by prepending a !:

!echo "Hello World!"

This would be a great feature to add to this core and, I guess, a fairly simple improvement.

[n-riesco]

The operator ! already exists in Javascript.
See #43 for more context.
If you still want to tread that path, see magicpatch.

Closing as duplicate of #43.